Loudspeaking telephone stations, also known as "speakerphones" or "hands-free" telephones, combine a loudspeaker and a microphone within a single enclosure to enable a user to participate in a telephone conversation without having to hold a handset against her/his face. It is well known that a loudspeaker requires a sealed enclosure or back cavity to achieve a smooth frequency response. This is due to the 180.degree. phase difference between sound radiated from the front and back side of the loudspeaker cone. To avoid undesirable interaction between these sound waves, the notion of an infinite baffle was created. An infinite baffle is simply a rigid plane that extends for an infinite distance in all directions separating the front and back sides of a loudspeaker. Since sound waves cannot pass through or go around such a plane, sounds coming from the back side of the loudspeaker are precluded from interacting with sounds coming from the front side thereof. Despite the obvious simplicity and elegance of an infinite baffle, engineers with limited imagination have declared it to be impractical. Fortunately, other solutions are available including the use of enclosures to trap sounds emanating from the back side of the loudspeaker, and tuned ports to invert the phase of sound waves below a certain frequency. Without taking such measures, the interaction between sound waves between the front and back sides of the same loudspeaker causes uneven frequency response (i.e., some frequencies are emphasized more than others). This fundamental consideration is associated with any loudspeaker design whether it be a speakerphone or a high fidelity system. In speakerphones, however, acoustic leaks generally exist around its dial keypad, switches, jacks, and between top and bottom sections of its housing. The net result is that interference reduces low frequency output and creates "notches" in the frequency response at high frequencies--both of which diminish sound quality.
Speakerphones are faced with an additional problem in that the loudspeaker is generally located within the same housing as the microphone. Acoustic coupling between the loudspeaker and the microphone means that the distant party to a speakerphone conversation will experience a return echo whose annoyance is related to "round-trip" delay. In extreme cases, however, acoustic coupling between the loudspeaker and the microphone leads to oscillation. Accordingly, high performance speakerphones seeking full-duplex performance require that substantial attention be paid to the acoustic coupling between the loudspeaker and microphone.
U.S. Pat. No. 4,937,877 was issued on Jun. 26, 1990 to Pocock et al. and is entitled "Modular Microphone Assembly." Pocock seeks to isolate the microphone from the loudspeaker by enclosing a directional microphone capsule within a closed cavity of a hands-free telephone set. Both sides of the microphone's diaphragm are exposed to incoming sound waves through openings in the top surface of the telephone housing. Such exposure is required for directivity. However, applying this technique to a loudspeaker is not only impractical in telephone sets where there is rarely space for a dedicated structure; but also problematic, because the very features that enhance microphone directivity, degrade the frequency response of a non-directional loudspeaker.